Aluminum-magnesium casting alloys



Sept. 25, 1956 c. B. WILLMORE 6 I I ALUMINUM-MAGNESIUM CASTING ALLOYSFiled Aug. 24, 1950 2 Sheets-Sheet 1 CHARTII SHOWING EFFECT OF so/eolvAND BEkYLL/UM As INTENS/F/ER FOQ THE G/A/N REF/NW6 0F TITAN/UM uv AL-MAGAuons HA vme 6.5 PER CENT MAGNESIUM ADD/flown METALS GRAIN DIAMETER INMILL/METERS PER CENT BORON //v 4110) INVENTOR. L Charles 5. Willmore emhflw azep Z6W$ M 147'? am e U/ Patented Sept. 25, 1956,

ALUM-MAGNESIUM CASTING ALLOYS Charles B. Wilimore, Newark, Ohio,assignor to William F. .lobhins, Incorporated, Aurora, 11]., acorporation of liiinois Application August 24, 1950, Serial No. 181,261

2 Claims. (Cl. 75-147) This invention relates to aluminum alloysconstituted with magnesium as the major alloying element, and, moreparticularly, it relates to new and improved aluminum-magnesium alloysfor fabrication into finished products by the casting technique. Thisapplication is a continuation-in-part of my copending application SerialNo. 71,015, filed on January 14, 1949, and entitled Aluminum-MagnesiumCasting Alloys, now Patent No. 2,564,044 issued on August 14, 1951.

Commercially, aluminum-magnesium casting alloys may be arranged into twodistinct groups. Cast alloys having a magnesium content ranging from 9to 12 percent by weight are responsive to heat treatment by which theirphysical properties are greatly improved. In this treatment, thealuminum-magnesium intermetallic compounds are put into solid solutionfrom which they are reprecipitated at room temperature in finely dividedform instead of the coarse crystals in which they existed in theoriginal casting. The major portion of reprecipitation takes placewithin a few days of aging whereby improved physical properties aredeveloped.

In the range of 3 to 9 percent magnesium, heat treatment has very littleeffect on the physical properties developed on casting. Alloys withinthis lower group form the subject matter of this invention. Theirphysical properties developed on casting are generally referred to asthe as cast properties. Within this group, further subdivision ispossible with respect to the method of casting; that is, casting may bemade into sand molds, hereinafter referred to as sand casting, or it maybe made into permanent molds, hereinafter referred'to as chill casting.Permanent mold or chill casting may rely entirely on gravitationalprinciples, or the use of positive pressure may be employed in fillingthe molds, as in die casting. A chief difference between the two typesof casting resides in the rate of heat transfer through the mold walls,it being greater in chill casting with the result that crystallizationand solidification are more rapid.

Chill casting usually has the effect of decreasing grain size of thecast alloys, especially when they are composed of an aluminum base.Inaluminum magnesium alloys, components, suchas magnesium, presentinquantities above their normal solid solubility limit at roomtemperature are retained in meta-stable condition of solid solutioninstead of precipi-tatingv out as in the slower cooling sand castingmethods. Ordinarily, these characteristics in a metalor alloy lead toimproved physical properties, but the reverse effects are obtained withaluminum-magnesium alloys; There is no satisfactory explanation for thedistinguishing deficiency in behavior of aluminum-magnesium alloysj't'odevelop superior properties responsive to finer grain size and increasedamount of magnesium in solid solution.

to my knowledge, has been able to nianufaoture ar a-1w- Per-. haps it isthe dominationof the coring effect. No one,

obtained by casing the same alloy in sand. An object of this inventionis to produce an aluminum-magnesium base alloy in which the usual lossof physical properties resulting from the coring effect in rapidfreezing is not encountered.

It is an object of this invention to produce an aluminum-magnesium alloywhich is not subject to the limitations of the prior art in that it canbe used for both chill and sand casting without substantial differencein physical properties.

Another object is to produce an aluminum-magnesium alloy for castinginto sand, refractory, or metal molds to provide a cast product havingimproved physical properties Without the need for any heat treatment.

A further object is to produce an aluminum-magnesium casting alloy thathas properties superior to any hereto fore obtained from analuminum-magnesium alloy of corresponding magnesium content by eithersand casting or by heat treatment; that has excellent tensile strengthand ductility without heat treatment; that is as resistant to corrosionas most of the aluminum-magnesium alloys, alloys which are distinguishedby their excellent corrosion resistance and high lustre; that hasoptimum machining properties; that acquires and retains a brilliantsurface responsive to simple polishing; and that develops highmechanical properties immediately upon cooling to room temperature,which properties do not change with age as compared with heat treatedcastings which develop equivalent tensile strength with age but with acorresponding loss in elongation or ductility such that the productultimately might become embr'ittled.

minum-magnesium alloy for chill casting which has physical propertiesthat are as high or higher than those A still further object is toproduce an aluminummagnesium alloy which is particularly adapted todevelop superior physical properties by chill casting although it may besuccessfully sand cast.

A still further object is to produce an aluminum alloy constituted with3' to 9 percent magnesium as the major alloying element and with othermetals in various new arrangements to provide for specific improvementin the physical characteristics of the cast alloy whereby excellentcombinations of tensile strength, yield strength, and elongation aredeveloped without resorting to expensive heat treatment, which is also adeterrent to the rate of production.

A. further object is to produce an aluminum-magnesium alloy whichembodies alloying principles differing from tho'se heretofore followedto provide for improved characteristics in the alloy and which has adecidedly added advantage in that it has less adhesion to the moldsurface inwhich it is cast and can more readily be removed from themold.

Briefly described, invention resides in alloying with aluminumandmagnesium, minor but important quantities of titanium, beryllium andboron to provide alloys having improved characteristics differing fromthose heretofore produced not only in composition but because ofalloying principles heretofore unrecognized in the production of new andimproved products. As previously pointed out, this invention is directedprimarily to aluminum-magnesium alloys for use in as cast condition and,therefore, is limited to less than 9 percent magnesium content, it beingunderstood that best properties are developed with magnesium present inthe range of 6 to 8.5 percent. Heretofore, the best aluminum alloyprepared by sand or permanent mold casting and having 6 to 8.5 per centmagnesium, have a tensile strength of 32,000pounds per square inch andelongation in order of 10 percent; whereas, by practicing my invention,an aluminum-magnesium alloy may be produced having as cast propertieswhich measure 42,000 pounds per square inch tensile and 15 percent ormore elongation, a combination of properties which exceeds thatobtainable with heat treated cast aluminum alloys of the same magnesiumcontent and is comparable in many instances alloys with higher magnesiumcontent.

To develop improved physical properties in metal a1- loys, eifort ismade to reduce the grain size by means which are not deleterious toothers of the more important physical characteristics. Boron, as well astitanium, molybdenum, and vanadium, has the reputation of grain refiningaluminum base alloys, but this reputation is predicated primarily on itsefliect with aluminum alloyed with copper and the like. By itself, boronis not a grain refiner in aluminum-magnesium alloys. This can best beillustrated with reference to Chart I showing that by the addition of0.001 percent boron, the grain diameter of the resultingaluminum-magnesium alloy is increased from 0.59 to 0.99 millimeter indiameter, and by the addition of 0.005 per cent boron, the graindiameter is increased still further to 1.00 millimeter.

Chart 1 shows the effect of boron and beryllium as intensifiers for thegrain refining of titanium in the aluminum-magnesium alloys having 6.5percent magnesium, and Chart 2 shows the elfect of titanium on thephysical properties of aluminum alloyed with 6.5 percent magnesium.

I have found that boron, which is not a grain refiner when added byitself to aluminum-magnesium alloys, nevertheless, acts as desired on analuminum-magnesium alloy, which has been grain refined as far aspossible with titanium, to refine the grain still further provided thatberyllium is also present; that is, boron and beryllium serve tointensify the grain refining efiect of titanium although neither boronnor beryllium alone has this intensifying effect on titanium. This isalso illustrated by Chart I drawn from the results secured from a largenumber of experiments. In the chart, lines B and D indicate that thegrain size increases with the addition of boron to an alloy in whicheither beryllium or titanium alone is present. On the other hand, linesE, F and G show that the grain size decreases from a relatively lowvalue as the amount of boron is increased when in the presence of bothtitanium and beryllium in the aluminummagnesium alloy.

In formulating with boron to secure the desired results, I prefer tolimit the use of boron to less than 0.01 percent by weight because itappears that aluminummagnesium alloys are incapable of retaining more insolution and that excess boron is precipitated out as an intermetalliccompound of boron, which does not appear to add to the physicalproperties of the alloy but instead becomes detrimental, especially ifthe amount of precipitation is excessive. For sand casting, it is bestto hold the beryllium content to less than 0.03 percent by weight but,preferably, in the range of 0.005 to 0.02 percent. For chill casting,beryllium content as high as 0.20 is useful, but it is most economicalto hold the beryllium content to less than 0.07 percent. In any event,more than 0.001 percent beryllium should be used.

It appears that maximum benefit of titanium is derived when it ispresent in amounts ranging from 0.10 to 0.2

. 4 percent. Larger quantities, up to, at least 0.40 percent titanium,may be used; however, it is probable that amounts in excess of 0.25percent do not remain dissolved in the alloy throughout its freezingrange and, therefore, can be of little additional benefit. Furthermore,it appears that as the liquid alloy passes through its freezing range,the excess titanium tends to form precipitates of intermetalliccompounds with other metals, making the liquid metal more sluggish tothe extent that excess titanium may be deterimental to the mechanicalproperties of the casting. In view of the above, I prefer to use lessthan 0.25 percent titanium, with best results being secured with amountsranging from 0.05 to 0.25 per cent titanium.

Grain size is an important factor in the determination of the physicalcharacteristics of an aluminum-magnesium alloy. Grain refinement leadsto improvement in tensile strength, yield strength, and elongation orductility, properties which spell the acceptability and commercialsuccess of the alloy in various applications. Grain refinement,therefore, is an important characteristic and the discovery of meanswhereby it may be eifected to control or improve other physicalproperties constitutes an important advance in metallurgicalcompositions, and the means by which it is secured suggests new alloyingprinciples. This I have accomplished with a new and improvedfive-component system of aluminum-magnesium, boron, titanium, berylliumwithin the limitations described.

From a practical standpoint, the five-component system embodyingfeatures of my invention has the added advantage that the definedcharacteristics apply to both sand casting and chill casting. This isunusual in aluminum-magnesium alloys because of the vast differencesthat exist in their crystallization whereby finer grain size and theretention of excess metals as solid solutions are characteristic ofchill casting. For most aluminum alloys, physical properties developedby chill casting are superior to those obtained by sand casting, but foraluminum-magnesium alloys, the reverse is more often true. This is bestillustrated by Table I which shows the physical properties determinedafter sand and chill casting. To the best of my knowledge, no oneheretofore has developed an aluminum-magnesium alloy which givesphysical properties by chill casting which are comparable to the samealloy cast in green sand.

TABLE I Ohil] Casting Sand Casting Method Method Mg Ti Be B PercentageAlloying Metals 6. 5 Ultimate Strength, Lbs./Sq. In 36,300 YieldStrength, Lbs/Sq. In 16, 900 Percentage Elongation A furtherillustration of the advantages of this invention and the ability toreverse the usual trend between sand and chill casting is as follows:

TABLE II Sand Casting Ohili Casting Exp.No. Mg Ti Be B Ult. YieldElong., Ult. Yield Elong.,

Str., Str., Percent Str., Str., Percent p. 5.1. p. s. i. p. s. i. p. s.i.

Formula A illustrates the usual trend in aluminummagnesium alloys-thephysical properties in chill casting falling oil? from the propertiessecured by the same alloy in sand casting. Formula B, which embodiesconcepts of this invention, begins to reverse the trend in that theproperties in sand or in chill casting are comparable. Formula C showswhat can be done for the properties in permanent mold or chill castingby bringing the beryllium and boron content within preferred ranges.

By other slight variations of percentages of these same elements Withinthe limitations prescribed, it is possible to achieve an alloy whereinthe properties developed by chill casting, especially in molds heated to600900 F., are still higher in many respects compared to those securedby the best sand cast alloys. In many instances, the same alloy may beused for sand casting and for chill casting interchangeably and stilldevelop excellent physical properties. A common formulation for use insuch two dissimilar casting processes is an achievement which has beenthe subject of concentrated research.

For chill casting, the boron content should be less than 0.01 percentbut more than 0.001 percent to be effective. Beryllium, in amounts up to0.05 percent, is very efiective, and excellent physical properties havebeen developed with as much as 0.2 percent, but because of its highcost, use beyond 0.07 percent may not be economical. Reasons previouslypointed out for keeping the titanium content below 0.40 percent and,preferably, below 0.25 but above 0.10 percent still hold true.

In production, the alloy may be compounded by the addition of themetallic component to molten aluminum maintained at least 100 degreesabove melting temperature. To the molten aluminum, the other elementsmay be added in any desirable order, conforming to acceptedmetallurgical practices limited to the production of an end producthaving the elements present in desired amounts and free of harmfulimpurities. In some instances, it is better to alloy with pure metals,while in other instances, additions may best be made as master alloys oras inorganic salts from which the metal may be made available and fromWhich benefit may be had of certain released gases and compositionswhich tend to remove impurities and gases from the melt. For example,beryllium may be incorporated as a master alloy with aluminum, andtitanium and boron may be added to advantage as inorganic salts.

Plus impurities.

It will be apparent from this description that I have conceived ofheretofore unknown alloying principles which have led to the inclusionof various alloying elements to produce aluminum-magnesium alloys havingcharacteristics far superior to those presently known, as produced bysand casting or chill casting With or without heat treatment. Ofconsiderable importance is the possibility of using the resultingcompositions interchangeably for casting in permanent molds or greensand without deleteriously affecting the physical properties.

Evident also is the fact that for the first time in aluminum-magnesiumalloys, elements may be incorporated for the purpose of increasing yieldstrength to a desirable high value Without the lowering of ultimatestrength and elongation. These and other concepts have led to theproduction of aluminum-magnesium alloys having considerable advantageover those heretofore produced.

It Will be understood that numerous changes may be made in the amountsof materials and methods of incorporation and fabrication into a castproduct Without departing from the spirit of myinvention, especially asdefined in the following claims.

I claim as my invention:

1. An aluminum base alloy for sand casting consisting essentially of 39percent by Weight magnesium, 0.0010.2 percent by weight beryllium,0.00l-less than 0.01 percent by weight boron, and 0.050.25 percent byweight titanium, the balance being aluminum.

2. An aluminum base alloy for chill casting consisting essentially of3-9 percent by weight magnesium, 0.00102 percent by weight beryllium,0.001less than 0.01 percent by weight boron, and 0.05-0.25 percent byweight titanium, the balance being aluminum.

References Cited in the file of this patent UNITED STATES PATENTS1,910,656 Tullis May 23, 1933 2,290,022 Bonsack July 14, 1942 2,369,213Cooper Feb. 13, 1945 2,463,021 Cooper Mar. 1, 1949 2,564,044 WillmoreAug. 14, 1951 OTHER REFERENCES Foundry Trade Journal, November 17, 1938,pages 373 and 374.

Metal Industry, July 25, 1947, page 71.

1. AN ALUMINUM BASE ALLOY FOR SAND CASTING CONSISTING ESSENTIALLY OF 3-9PERCENT BY WEIGHT MAGNESIUM, 0.001-0.2 PERCENT BY WEIGHT BERYLLIUM,0.001-LESS THAN 0.01 PERCENT BY WEIGHT BORON, AND 0.05-0.25 PERCENT BYWEIGHT TITANIUM, THE BALANCE BEING ALUMINUM.